I. Field of the Invention
The invention relates to a process for recovering C.sub.3 -C.sub.4 components absorbed by or entrained in amine absorbents during absorption removal of H.sub.2 S from liquid C.sub.3 -C.sub.4 streams.
II. Background of the Invention
Hydrocarbon streams containing or comprising significant quantities of C.sub.3 -C.sub.4 components are common process streams in petroleum refineries and petrochemical plants. Crude processing, for example, produces a stream of mostly saturated C.sub.3 -C.sub.4 distillate, while fluid catalytic cracking units produce a byproduct which comprises mostly C.sub.3 -C.sub.4 unsaturates. These C.sub.3 -C.sub.4 streams typically contain minor amounts, e.g., 0.5 to 10 percent by weight, or less, of H.sub.2 S, a contaminant which can seriously interfere with further processing or use of the streams. Accordingly, treatment of C.sub.3 -C.sub.4 streams to remove H.sub.2 S or reduce its concentration to a level tolerated by conversion catalysts or subsequent users is standard practice.
Commonly, the H.sub.2 S is removed from such streams by procedures in which the C.sub.3 -C.sub.4 stream is intimately contacted, under pressure sufficient to maintain the stream as a liquid, with an "absorbent" or solution comprising an amine, such as an alkanolamine. The "absorption" of the H.sub.2 S from the liquid C.sub.3 -C.sub.4 is considered to occur by a mechanism in which the H.sub.2 S purportedly forms a compound with the amine at lower temperatures, this compound being readily dissociated at higher temperatures, thereby allowing "desorption" of the H.sub.2 S and its further processing. Whatever the validity of the supposed mechanism, absorbency or absorption terminology has become firmly established and employed with respect to amine H.sub.2 S removal, and is so employed herein. As indicated, prior art absorption of H.sub.2 S from liquids and gases is well known, as exemplified in U.S. Pat. No. 4,278,621, U.S. Pat. No. 4,297,329, both by Sigmund et al, and in the literature, such as in Gas and Liquid Sweetening, by R. N. Maddox, Campbell Petroleum Series, 1974.
During the amine absorption treatment of liquid C.sub.3 -C.sub.4 streams to remove H.sub.2 S, the slight but significant solubility of the liquid C.sub.3 -C.sub.4 composition(s) in the amine absorbent at the pressures employed and the possible physical entrainment of the C.sub.3 -C.sub.4 liquid in the amine may pose significant problems. A minor amount, e.g., 20 to 250 ppm, by weight, of C.sub.3 -C.sub.4 hydrocarbon is commonly found in the amine exiting the absorber for regeneration, resulting in a significant amount of the hydrocarbon in the amine recovery system. In at least one conventional processing scheme, C.sub.3 -C.sub.4 components in the amine have simply been allowed to flash at reduced pressure in a flash zone or drum before regenerating the amine, the flashed C.sub.3 -C.sub.4 gas simply being sent to flare. This procedure, however, represents a net loss of C.sub.3 -C.sub.4 hydrocarbon and increases the volume of material which must be handled by the flare. Accordingly, a need has existed for procedure for dealing with C.sub.3 -C.sub.4 values in the rich amine that avoids the necessity for significant flaring. The invention addresses this need.